Chapter 12 DNA structure

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Chapter 12DNA structure
Watson, Crick, and Franklins crystallography
image
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How did scientists know that DNA carries the
information for life?

• Many thought that proteins instead of DNA were
  inherited from parent to offspring.
• Scientists (Griffith, Hershey, and Chase) showed
  that DNA actually carries the blueprint.

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Griffiths experiment
Heat-killed, disease-causing bacteria (smooth
colonies)
Harmless bacteria (rough colonies)
Control(no growth)
Harmless bacteria (rough colonies)
Heat-killed, disease-causing bacteria (smooth
colonies)
Disease-causing bacteria (smooth colonies)
Dies of pneumonia
Dies of pneumonia
Lives
Lives
Live, disease-causingbacteria (smooth colonies)
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Hershey and Chases experiment
Bacteriophage with phosphorus-32 in DNA
Phage infectsbacterium
Radioactivity inside bacterium
Bacteriophage with sulfur-35 in protein coat
Phage infectsbacterium
No radioactivity inside bacterium
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After determining the connection between DNA and
genetics, figuring out the structure of DNA
became important.

• Each strand of DNA is made of repeating
  nucleotide units.
• A nucleotide is either a purine or a pyrimidine.

Purines
Pyrimidines
Adenine
Guanine
Cytosine
Thymine
Phosphate group
Deoxyribose
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Watson and Crick won the Nobel Prize for figuring
out the DNA is actually 2 strands twisted
together, called a double helix.
Nucleotide
Hydrogen bonds
Sugar-phosphate backbone
Key Adenine (A) Thymine (T) Cytosine (C) Guanine
(G)
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Double helix structure
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Bacterial DNA
Chromosome
E. coli bacterium
Bases on the chromosome
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Eukaryotic DNA
Nucleosome
Chromosome
DNA double helix
Coils
Supercoils
Histones
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DNA the genetic basis of life

• Do you notice any commonalities in the
  percentages of adenine, thymine, guanine, and
  cytosine? Between species?

Source of DNA A T G C
Streptococcus 29.8 31.6 20.5 18.0 Yeast 31.3 32.9
18.7 17.1 Herring 27.8 27.5 22.2 22.6 Human 30.9 2
9.4 19.9 19.8
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Chromosome packing
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Before replication begins

• Chromosomes must be unwound back to chromatin
  through the removing of histone proteins.

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Enzymes for DNA replication

• Helicase
• Single stranded binding protein
• Topoisomerase
• DNA polymerase
• Ligase

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Helicase

• Breaks hydrogen bonds between nitrogen bases of
  nucleotides
• Opens double helix starting at origin of
  replication

Helicase
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Topoisomerase SSBP

• SSBP (single stranded binding proteins)
  stabilize open helix because DNA is stable when
  double-stranded
• Topoisomerase helps to relieve the tension
  created when helicase creates replication bubble

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Topoisomerase and SSBP
Topoisomerase
SSBP
SSBP
Topoisomerase
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DNA polymerase

• Builds DNA polymer according to Chargaffs base
  pairing rules
• A T
• C G
• DNA polymerase also edits to check for mistakes
  in the DNA.

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DNA polymerase
Leading strand
A T T A C A - 3 T A A T G T - 5
G
C
T
A
5 A A A T T C G T
A T
3 T T T A A G C A
T A
Lagging strand
C G
G C
T A A T G T - 5
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DNA replication in 5 to 3 direction
Original strand
DNA polymerase
New strand
Growth
DNA polymerase
Growth
Replication fork
Replication fork
New strand
Original strand
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Leading vs. Lagging strands

• Leading strand made continuously
• Lagging strand made in fragments (called Okazaki
  fragments)

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Ligase

• Reforms bonds between parts of the nucleotides
  and between 2 nucleotides.

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DNA synthesis overview
A B
C D
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Click on image to play video.
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What do we want to know from the code that DNA
carries?

• The DNA structure was discovered just over 50
  years ago.
• What can DNA tell us?
• DNA codes for proteins, and proteins are the
  foundation for all metabolic activity.

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DNA song to Row your boat

• We love DNA
• Made of nucleotides
• Sugar, phosphate, and a base
• Bonded down one side.
• Adenine and thymine
• Make a lovely pair
• Cytosine without guanine
• Would feel very bare

• Oh-h-h, de-oxy-ri-i-bo
• Nu-u-cleic acid
• RNA is ri-i-bo
• Nu-u-cleic acid
• (Repeat)